Aerated lagoons

WASTE WATER TREATMENT BY AERATED LAGOONS , AN EFFECTIVE METHODOLOGY FOR WASTE WATER TREATMENT ALONG WITH ITS DESIGN FUNDAMENTALS AND WITH ADVANTAGES AND DISADVANTAGES .

1. AERATED LAGOONS SUBMITTED BY- ANKIT YADUVANSI 2k16/ENE/03

2. INTRODUCTION An aerated lagoon or aerated basin is a holding and/or treatment pond provided with artificial aeration to promote the biological oxidation of wastewaters . In contrast of waste stabilisation ponds where the oxygen required for stabilising the organic matter is furnished by algae, the oxygenation is provided in Aerated lagoons by mechanical surface aerators installed on floats or rates or fixed platforms.

3. TYPES OF AERATED LAGOONS Suspension mixed lagoons, where there is less energy provided by the aeration equipment to keep the sludge in suspension . Facultative lagoons, where there is insufficient energy provided by the aeration equipment to keep the sludge in suspension and solids settle to the lagoon floor. The biodegradable solids in the settled sludge then degrade as in an anaerobic lagoon .

4. METHODS OF AERATING LAGOONS Motor-driven submerged or floating jet aerators. Motor-driven floating surface aerators Motor-driven fixed-in-place surface aerators Injection of compressed air through submerged diffusers.

5. PROCESS DESCRIPTION A treatment plant for domestic wastewater should consist of a fine screen, two aerated lagoons in series and at least one polishing lagoon. First, a fine screen removes coarse matter. A paved inlet zone in the first lagoon allows settlement of heavy sludge and grit. A scum baffle separates inlet zone and lagoon and prevents floating matter from entering the lagoon. Floating matter has to be removed manually once or twice a week with a rake.

6. According to our experience, sludge in the inlet zone of the first aerated wastewater lagoon has to be removed at regular intervals of several years. Liquid manure-vacuum-tankers are used to pump off the sludge. Dissolved and suspended organic pollutants are distributed in the whole first lagoon. Decomposable organic matter should mainly be stabilized aerobically to avoid odours or anaerobic sludge rising to the water surface . The treatment process in aerated lagoons is similar to the natural process in flowing water bodies. Biological degradation of pollutants is based on attached growth.

7. The biofilm needs continuous supply with oxygen and organic pollutants. In addition to sufficient oxygen transfer, circulating flow in the whole water body and effective mixing is required. The circulation of wastewater and dissolved oxygen ensures optimal conditions for aerobic growth at the lagoon bottom. Thus, organic pollution is highly reduced and dead zones can be avoided. Polishing lagoons further improve the water quality. In particular, suspended light flocs of biomass settle down.

8. DESIGN FUNDAMENTALS Aerated lagoons for municipal wastewater are designed according to the volume load and the retention time. Volume load may not exceed 20 g BOD5/(m³.d). This correlates to a specific volume greater than 3.0 m³/ p.e. The retention time may not be less than 5 days even if a combined sewer system is used and infiltration rates are high.

9. Mixing requirements are the design basis for the second stage. According to the shape of the lagoons a power density between 1.3 - 2 W/m³ is sufficient. The aforementioned design criteria ensure sufficient oxygen transfer even at times of increased oxygen uptake, for example in early summer or at peak load. In times of low oxygen demand intermittent operation can save energy.

10. Two aerated stages in series are advantageous with regard to permanent high effluent quality. The first stage should have approx. 60%, the second stage approx. 40% of the total volume. A two-stage lagoon system performs much better than a one-stage design with reduced volume load . Polishing lagoons should provide 1 – 2 days retention time. This corresponds to a specific lagoon volume of 0.3 - 0.5 m³/ p.e.

11. DESIGN OF THE LAGOONS Aerated lagoons are often designed in rectangular shape. To enable a good circulation and mixing at low power demand, the corners should be rounded and the slope should not exceed a 2:1 (width to height) ratio . It is also possible to adapt the shape of the lagoon to the surrounding ground. However, in this case more aerators of smaller size may be needed.

12. The depth of the lagoons should range between 2.00 - 3.00 m; mostly a water depth of 2.50 m is chosen. The angle of the slope corresponds to the local soil conditions. Normally it is 1:1.5 to 1:2. Natural or clay lining should be used wherever possible. Plastic lining should be applied only in highly permeable soil in water protection areas . The inlet zone of the first aerated lagoon should have a depressed floor level and it should be reinforced or paved, so that settled sludge and grit can easily be pumped off without risk of damage at normal waterlevel .

13. To prevent damages caused by muskrats, plants or erosion the slope should be reinforced at the height of normal water level. The shape of the final clarification or polishing lagoon may vary in a wide range. However, short circuiting has to be avoided. The depth of the polishing lagoon should be approx. 1.20 m.

14. ADVANTAGES Lagoon systems can be cost-effective to design and construct in areas where land is inexpensive. They use less energy than most waste water treatment methods. They are simple to operate and maintain and generally require only part-time stuff. They can handle intermittent use and shock loadings better than many systems, making them a good option for campgrounds, resorts, and other seasonal properties.

15. DISADVANTAGES Lagoon systems require more land than other treatment methods. They are less efficient in cold climates and may require additional land or longer detention times in these areas. Odour can become a nuisance during algal blooms or with anaerobic lagoons and lagoons that are inadequately maintained. Unless they are property maintained, lagoons can provide a breeding area for mosquitoes and other insects.

16. SUMMARY Aerated lagoons are an efficient and cost-effective system for primary and secondary wastewater treatment in small communities. They can be integrated very well into the surrounding landscape. If appropriate aerators are used the power input is similar to a comparable activated sludge plant. In addition to sufficient oxygen transfer the aerators have to provide for mixing and circulation. Expenditures for maintenance and operation are restricted to a minimum

17. AERATED LAGOON IN GERMANY